Liquid Systems Based on Tetra(n-butyl)phosphonium Acetate for the Non-dissolving Pretreatment of a Microcrystalline Cellulose (Avicel PH-101)

A non-dissolving pretreatment consisting in the direct contact of cellulose and the ionic liquid tetra(n-butyl)phosphonium acetate, or its fluid mixtures with other phosphonium ionic liquids or with molecular liquids such as ethanol or DMSO, causes a reduction in the crystallinity of the popular microcrystalline cellulose-type Avicel PH-101 under mild conditions. At the same time, the degree of polymerization and the thermal stability of the pretreated Avicel remain essentially unaltered with respect to the untreated Avicel. The diminution of the crystallinity has been related to the increase of the reactivity of the pretreated Avicel samples via analysis of the kinetics of their enzymatic hydrolysis. For selected samples, this improved reactivity has been confirmed through their effective carboxymethylation under a simplified and milder reaction procedure.

The peak at 2.5 ppm corresponds to the residual proton signal of the perdeuterated solvent. Figure S4. 13

Calibration of the ATR-FTIR spectroscopy method for the determination of the degree of substitution (DS) of carboxymethylated cellulose samples
To calibrate the method based on the carboxylate vs. methylenic/methinic absorbance ratio (RCM) measured by ATR-FTIR spectroscopy, the DS for a series of carboxymethylcellulose samples in sufficiently large amounts were determined by acid-base back-titration, and then correlated to the RCM values obtained by ATR-FTIR. Such carboxymethylcellulose comprised a commercial one (sodium carboxymethylcellulose, Mw ~ 250,000, degree of substitution 1.2, Sigma-Aldrich) and a range of samples prepared by standard procedures varying solvent composition (2-propanol/water ratios) and/or reaction time -See Table S4 and paragraphs below. This covered a wide DS range (0-1.2). The resulting linear regression correlating DS and RCM, shown in Figure S10, presents a satisfactory coefficient of correlation: R 2 = 0.981.    Table S4.
Carboxymethylation. The carboxymethylation of cellulose samples in sufficiently large amount for titration analyses was performed by a procedure adapted from Pushpamalar et al. (2006).
In a typical carboxymethylation, a cellulose sample (0.500 g) was weighted in a 25 mL roundbottomed flask, and it was suspended in 2-propanol (Scharlau ACS Basic, ≥99.5 %, 10 mL). A volume of 1.0 mL of a 20 % w/v aqueous solution of sodium hydroxide (Sigma-Aldrich, ≥98 %) was then added dropwise under magnetic stirring, and the resulting suspension further stirred at 500 rpm for 1 h at room temperature. The flask was then immersed in an oil bath preheated at 40 °C and the suspension stirred at 700 rpm for 2 min. Sodium chloroacetate (Sigma-Aldrich, for synthesis, 0.600 g) was added and the suspension stirred at 700 rpm for the desired time (0.5, 1 or 3 h). After reaction, the flask was taken out of the bath, allowed to cool down to room temperature and the solid separated by filtration through a sintered glass funnel (pore size: 3), washed with 2propanol (3 × 3 mL) and air-dried by suction. The collected solid was suspended in methanol (Merck, ACS reagent, ≥ 99.9%, 30 mL) and the stirred mixture neutralised until pH ≈ 6, as measured using pH indicator paper, by adding the required amount of glacial acetic acid (Scharlau, extrapure). The solid was separated by filtration on a sintered glass funnel (pore size: 3), washed with methanol (5 × 3 mL) and dried under an air stream by suction. The final sodium carboxymethylcellulose (Na-CMC) samples had a white colour and their texture ranged from powdery solids to slightly sticky flakes depending on their degrees of substitution.

S9
Acidification. Conversion of Na-CMC samples into their acid form was carried out by adapting procedures described in the literature (Eyler et al., 1947;Pushpamalar et al., 2006). The procedure varied slightly depending on whether the CMC was commercial or had been synthesised from cellulose in our laboratories.
In a typical acidification procedure for a sample prepared from cellulose, Na-CMC (0.  Figure S11).
Before titration, the H-CMC was placed in a Petri dish and dried in an oven at 105 °C for 1 h or until constant weight. The resulting weight was taken as its dry weight for DS quantification.
Acidification of commercial Na-CMC was performed under a more diluted and less aqueous regime to avoid gel formation, a phenomenon which seriously hinders solid recovery. In a typical procedure, Na-CMC (1.00 g) was dispersed in methanol (Merck, ACS Reagent, ≥99.9 %, 200 mL) in a 500 mL round-bottomed flask connected to a condenser. Nitric acid (J. T. Baker, 65 %, 2.0 mL) was added dropwise, the resulting suspension heated in an oil bath at 80 °C and stirred at 1000 rpm until boiling, then stirred for 5 min at 1000 rpm while boiling, and taken out of the oil bath while cooling down to room temperature under stirring. The solid was separated by filtration through a sintered glass funnel (pore size: 3), washed with methanol (5 × 10 mL), further washed with a methanol/water mixture (80:20 v/v, ca. 200 mL) until the pH of the liquor was >6, as detected by using pH indicator paper, and air-dried by suction. As above, the resulting solid was identified as the acid form of carboxymethylcellulose (H-CMC) based on ATR-FTIR spectroscopy. Before titration, the H-CMC was placed in a Petri dish and dried in an oven at 105 °C until constant weight.
The resulting weight was taken as its dry weight for DS quantification.
Titration. The DS of CMC samples was accurately determined by an acid-base back-titration method in aqueous solution consisting of a prior basification of H-CMC samples using a sodium S10 hydroxide solution, and the titration of the remaining free [OH] − by a hydrochloric acid solution.
The difference between such remaining [OH] − and that of a blank titration of an equal amount of sodium hydroxide (without H-CMC) is used to determine the amount of moles of acid in the CMC sample.
In a typical back-titration procedure, an accurately weighted sample of one of the prepared H-CMC materials (ca. 0.3000 g, corrected to only account for its dry weight, as described above) was dispersed in deionized water (60 mL Table S4. S11 Figure S11. ATR-FTIR spectra of sodium carboxymethylcellulose (Na-CMC) prepared from Avicel after 3 h reaction time, as for Entry 3 in Table S4, as compared to its acid form (H-CMC) prepared for determination of DS by titration. The effective conversion from sodium to protonic form is confirmed by the disappearance of the asymmetric carboxylate stretching band (ca. 1590 cm −1 ) and the appearance of the carboxylic (C=O) stretching signal (ca. 1730 cm −1 ).

References:
Eyler, R. W., Klug, E. D., & Diephuis, F. (1947 Table 1 of the main manuscript.  Table 1 of the main manuscript.  . The spectrum of raw Avicel is also included for comparison. The extent of carboxymethylation can be observed by the increasing signal at ca. 1590 cm −1 . Normalisation of absorbance was performed at the area of ATR artifacts (ca. 2150 cm −1 ) for a more realistic comparison.